This invention relates to apparatus for mixing fluids.
In, for example, the calibration of analytical instruments it is conventional to employ accurately preformed gas mixtures supplied under pressure in suitable containers (ie. gas cylinders). Such mixtures can be formed very accurately by manufacturers of industrial gases using gravimetric methods which require expensive equipment. It is uneconomic for the user of the gas mixture to make his own mixture by the methods employed by manufacturers of industrial gases. Accordingly, attempts have been made to produce for use on-site gas mixers which operate on the principle of passing the component gases through calibrated flow restrictors (or impedances) and forming a mixture whose composition depends on the ratio of the flow rates of the respective gases through the flow impedances. The ability to form gas mixtures accurately by such a technique depends, it has been believed, to a substantial extent on maintaining the gases to be mixed at substantially the same constant pressure upstream of the flow restrictors. To this end, pressure regulators are employed, and the gases to be mixed are typically supplied to the respective regulators at the same pressure. In practice, however, there are difficulties in obtaining a constant output pressure from a conventional regulator if there is a varying downstream flow resistances and thus special arrangements need to be adopted in order to solve this problem. (See for example U.S. Pat Nos. 3,493,005 and 3,762,427).
Another problem associated with the kind of gas mixer described above is the choice of flow resistor. It is possible to provide a series of orifice places with different sized orifices, or a series of needle valves of different dimensions from one another. However, it can be relatively expensive to make such flow resistors, and, moreover, considerable problems arise should one such flow resistor become clogged with a piece of solid material. One commercial gas mixer employs a number of stainless steel capillary tubes which are brazed or welded into a suitable holder. (The gas flow meter described in U.S. Pat. No. 3,487,688 shows a similar arrangement of capillary tubes.) Should one such capillary become blocked, replacement of it is a time-consuming operation involving breaking of the welded or brazed join which holds the defective capillary into the holder, and then the making of a new join.
Problems that arise in the design and construction of gas mixers have been described above. Analogous problems arise in the mixing of liquids.
It is an object of the present invention to provide, for use in a fluid mixer, flow restricting apparatus which may readily be kept in good working order. It is a further object of the present invention to provide relatively compact flow restricting apparatus of relatively simple construction for use in the fluid mixer.
It is a yet further object of the present invention to provide a fluid mixer including flow restricting apparatus that is relatively compact and of relatively simple construction, and that is readily demountable to enable individual flow restrictor elements to be replaced if needs be.